Arterio Venous Malformation MRI

Clinically a young patient with right sided sudden onset focal neurological deficit. 

Here is his MRI Brain Axial T2w images with non contrast 3D TOF MR Angiogram of Brain.

This MRI study of Brain show a focal tightly packed cluster of serpigenous flow voids giving so called 'bag of black worms' appearance of an Arterio Venous Malformation in left medial occipital lobe and on surface of left temporal lobe. Left PCA and cortical branches from left MCA inferior division appears to be the feeder.

Related post : Arterio venous malformation


Arterio Venous Malformation (AVM)

A high flow vascular malformation with arterio venous shunting, no intervening capillary bed.

Imaging wise diagnostic clue is tightly packed mass of enlarged vascular channels.
CT:
Plain CT is insensitive for small aneurysms.
Contrast enhanced CT study is must. Mass of tightly packed iso to hyperdense serpentine vessels with strong enhancement on post contrast. Calcification seen in ~20-30%.
MRI:
Investigation of choice, typical imaging findings even on non contrast study. Most preferred sequence is T2w images.
A focal intra axial 'bag of black worms' due to flow voids on T2w images. Signal on T1w images varies with flow rate, direction, presence of hemorrhage and blood degradation products. Strong enhancement noted on post contrast T1w images. FLAIR images may show adjacent parenchymal hyper intensity due to an associated odema or Gliosis. Low signal intensity hemosderin staining on T2*GRE may be seen if hemorrhage. Dw images are often non contributory.
MR Angiography and Venogram to depict feeder and venous drainage.
DSA:
Digital Substraction Angiography is Gold standard, delineates internal angio architecture.
Depicts three components of AVMs - enlarged arteries, Nidus of tightly packed vessels, Draining veins (AV shunt with early appearance of contrast in enlarged veins).
In ~ 20-30% of cases AVM has dual arterial supply (Dural, Pial). Dural supply of AVM occurs through leptomeningeal anastomosis or transdural anastomoses with normal cortical arteries. Identification of transdural anastomoses is important as it decides treatment plan, embolisation vs surgery.

In CNS, AVMs may occur anywhere in Brain and Spinal cord.
In Brain, in ~85% are in supra tentorium and ~ 15% in posterior fossa.
In ~98% of cases are solitary and sporadic lesions. Rarely multiple AVMs, are usually syndromic.
Size is variable from few centimeters to Gaint occupying complete left hemi cranium.
Associated abnormalities are aneurysm on feeding artery 10-15%, intranidal"aneurysm in > 50%.

Vascular "steal" may cause ischemia in adjacent brain



Pathogenesis: 
AVMs have dysregulated angiogenesis, undergo continued vascular remodeling.
• Vascular endothelial growth factors (VEGFs), receptors mediate endothelial proliferation, migration
• Cytokine receptors mediate vascular maturation, remodeling.

Genetics: 
Sporadic AVMs have multiple up-, down-regulated genes. Homeobox genes such as Hox D3 and B3 involved in angiogenesis may malfunction.
Syndromic AVMs (2% of cases). Multiple AVMs in HHT 1 (endoglin gene mutation). Cerebrofacial arteriovenous metameric syndromes (CAMS) have orbit/maxillofacial + intracranial AVMs.

Histo pathology
Gross specimen is wedge-shaped, compact mass of tangled vessels.
Microscopic features have wide phenotypic spectrum
Feeding arteries usually enlarged but mature (may have some wall thickening)
Enlarged draining veins (may have associated varix, stenosis)
Nidus
• Conglomeration of numerous AV shunts ("micro AVFs")
• Thin-walled dysplastic vessels (no capillary bed)
• Disorganized collagen, variable muscularization
• Lack subendothelial support
• Loss of normal contractile properties
• No normal brain (may have some gliosis)
Perinidal capillary network (PDCN) -  a nidus surrounded by dilated capillaries in brain tissue 1-7 mm outside nidus border, larger than normal capillaries, connects both to nidus, feeding arteries/draining veins, surrounding narrowed brain vessels

Spetzler-Martin scale
Size
• Small <3 cm="1</p">• Medium 3-6 cm = 2
• Large > 6 cm = 3
Location
• In "noneloquent" area = 0
• If involves eloquent brain = 1
Venous drainage
• Superficial only = 0
• Deep = 1
Sum of above estimates surgical risk

Clinical Presentation
Most common signs/symptom is Headache with hemorrhage 50%, Seizure 25%, Focal neurologic deficit 20-25%
Often Young adult with spontaneous non traumatic ICH. M = F

Prognosis
All brain AVMs are potentially hazardous
Risk of first hemorrhage is lifelong, rises with age (2-4%/year, cumulative)
Vast majority will become symptomatic during patient's lifetime
Spontaneous obliteration rare < 1% of cases

Treatment
Embolization, stereotaxic radiosurgery, microvascular surgery

Reference: Osborn Diagnostic Imaging.